You can get an OPML file with your YouTube subscriptions by running this script on the console while you are logged in at YouTube's main page. The downloaded file can be imported at any invidious instance.

export-subscriptions.js

function downloadOPML(opml) {
    // Create a new anchor element
    const anchor = document.createElement('a');

    // Set the anchor's download attribute
    anchor.download = 'subscriptions.opml';

    // Set the anchor's href attribute to the OPML data
    anchor.href = `data:text/xml;charset=utf-8,${encodeURIComponent(opml)}`;

    // Click the anchor element to trigger the download
    anchor.click();
    console.log("Downloaded OPML file");
}


async function getChannelId(url) {
    let channelId = url.split('/').pop();

    if (!url.startsWith('https://www.youtube.com/channel/')) {
        // Fetch the source code of the URL
        const response = await fetch(url);
        const html = await response.text();

        // Parse the source code into a DOM tree
        const parser = new DOMParser();
        const doc = parser.parseFromString(html, 'text/html');

        // Find the <link> element with the correct attributes
        const linkElement = doc.querySelector("link[rel='canonical'][href^='https://www.youtube.com/channel/']");
        //      console.log (linkElement)
        // Extract the channel ID from the href attribute
        const href = linkElement.getAttribute('href');
        channelId = href.split('/').pop();
        console.info(`Found channel ID ${channelId} for URL ${url}`);
    }

    return channelId;
}

var start = async function () {
    // Firefox requires that the slide out is visible to get the list of
    // subscriptions
    if (getShowMoreElement() === undefined) {
        // click on hamburger menu to show slide out
        document.querySelector("#guide-icon").click();
        while (getShowMoreElement() === undefined) {
            await new Promise(r => setTimeout(r, 500));
        }
    }

    getShowMoreElement().click();

    let subscriptions = Array.from(document.querySelectorAll(
        "a#endpoint")).filter(
            ytTextElement => ytTextElement.innerHTML.includes(
                "style-scope ytd-guide-entry-renderer no-transition")).map(elem => elem.title + "\t" +
                    elem.href);

    subscriptions.sort((a, b) => a.localeCompare(b, undefined, { sensitivity: 'base' }));

    function getShowMoreElement() {
        let subscriptionsPanel = document.querySelectorAll("ytd-guide-section-renderer")[1];
        if (subscriptionsPanel === undefined) return undefined; // it's still loading

        let subscriptionsPanelInner = subscriptionsPanel.querySelector("#items > ytd-guide-collapsible-entry-renderer");

        // it's loaded and there are enough subscriptions to show the "Show
        // more" button
        if (subscriptionsPanelInner !== null) {
            return subscriptionsPanel.querySelector("#items > ytd-guide-collapsible-entry-renderer")
                .querySelector("#endpoint");
        } else {
            // it's loaded, but there are just a few subscriptions it doesn't
            // really matter what we click on
            return document.createElement("foo");
        }
    }

}

async function main() {
    start();
    // Get all channel elements
    let subscriptions = Array.from(document.querySelectorAll(
        "a#endpoint")).filter(
            ytTextElement => ytTextElement.innerHTML.includes(
                "style-scope ytd-guide-entry-renderer no-transition")).map(
                    elem => [elem['title'] = elem.title, elem['url'] = elem.href]
                );
    console.log("Channels obtained: "+subscriptions.length)

    const channelData = [];

    // Iterate over the channels
    for (const channel of subscriptions) {
        // Extract the title and URL of the channel
        const title = channel[0];
        let url = channel[1];

        // Convert the URL to the correct format if necessary
        url = await getChannelId(url);
        url = `https://www.youtube.com/feeds/videos.xml?channel_id=${url}`;

        channelData.push({ title, url });
    }
    console.table(channelData)
    // Generate the OPML document
    const opml = `
  <opml version="1.1">
    <body>
      <outline text="YouTube Subscriptions" title="YouTube Subscriptions">
        ${channelData.map(({ title, url }) => `
          <outline text="${title}" title="${title}" type="rss" xmlUrl="${url}" />
        `).join('')}
      </outline>
    </body>
  </opml>
  `;
    downloadOPML(opml);


}
main();

Inspired by alexyorke/youtube-subscriptions-exporter